1. Field of the Invention.
This invention relates in general to telecommunications, and more particularly to a method and apparatus for calibrating a .DELTA.-.SIGMA. modulator quantizer to substantially replicate relative mismatches in the multi-bit digital-to-analog converter in the transmit channel of the modulation system.
2. Description of Related Art.
Global communications continues to demonstrate rapid growth rates. As more people become accustomed to the convenience of electronic mail, web-based facsimile transmission, electronic commerce, telecommuting and high-speed Internet access, the demand on the telecommunications industry to provide adequate bandwidth to provide this type of service also increases. The growth in the number of people using electronic communications will only increase as the price of Internet access and Internet access devices such as personal digital assistants (PDAs), computers, etc. decreases.
Today, copper telephone lines service almost all voice traffic and most of the Internet traffic. However, as content rich applications continue to grow, both public and private copper access networks are being challenged. The local portion of the enterprise becomes a major challenge for access providers. To take advantage of the increasingly popular innovations in telecommunications technology, additional telephone lines are being installed in private residences and businesses.
Although analog modems have managed to stretch their potential speed to 56 kilobits per second (kbps), small-office/home-office (SOHO) customers need far greater Internet bandwidth to accommodate multimedia applications ranging form three-dimensional web sites to video conferencing. Analog modems cannot deliver the necessary bandwidth and, therefore, have reached the end of their usefulness.
In response to these developments, communications companies are responding with a variety of digital access solutions, all variants of Digital Subscriber Line (DSL) technology. These DSL technologies differ dramatically in their abilities to address major SOHO applications and the requirements of telephone companies.
DSL technologies are transport mechanisms for delivering high-bandwidth digital data services via twisted-pair copper wires. These copper wires provide the cabling between the telephone company's central offices and subscribers. DSL technology is a copper loop transmission technology that solves the bottleneck problem often associated with the last mile between Network Service Providers and the users of those network services. DSL technology achieves broadband speeds over ordinary phone wire. While DSL technology offers dramatic speed improvements (up to 7+Mbps) compared to other network access methods, the real strength of DSL-based services lies in the opportunities driven by multimedia applications required by today's network users, performance and reliability and economics.
Without such transport mechanisms, subscribers would have to rely on T.sub.1 (1.5 Mbps) or E1 (2.0 Mbps) service, which requires the phone company to install expensive new cabling to every location that wants high-speed digital service. The installation costs make T1/E1 service expensive.
The original DSL service was ISDN DSL (ISDL), which was defined in the late 1980s. ISDL provides 160 kbps rates over a single twisted-pair at ranges up to 18,000 feet from the telephone company's central office. While this service has been deployed in many homes and small businesses around the world, the demands of multimedia applications are already challenging IDSL's bandwidth.
Asymmetric Digital Service Line (ADSL) is currently being embraced by residential web surfers for its ability to quickly download music and video files. ADSL refers to modem technology that transforms twisted copper pair (ordinary phone lines) into a pipeline for ultra fast Internet access. As the name suggests, ADSL is not asynchronous transmission, but rather asymmetric digital transmission, i.e., ADSL transmits more than 6 Mbps (optionally up to 8 Mbps) to a subscriber, and as much as 640 kbps (optionally up to 1 Mbps) in the other direction.
ADSL has the ability to increase normal phone line capacity by 99% via a digital coding technique. This extra capacity means that one could simultaneously assess the World Wide Web and use the telephone or send a fax. A user of this technology could have uninterrupted Internet access that is always on-line. This technology also has the potential to be a cost-effective solution for residential customers, telecommuters and small business.
Still, there is a need for symmetric high-speed connection. For example, small businesses have become increasingly dependent on sophisticated voice and data products and services for competing against larger corporations. Until now, the cost of providing small businesses with professional telephony and data services was prohibitive. However, integrated access and virtual public branch exchanges (PBXs) are providing small businesses with voice mail, high-speed Internet access, multiple business lines and sufficient capabilities for telecommuters.
As mentioned above, symmetric services were traditionally delivered by T1 and E1 lines. Within the DSL family, HDSL has long been used to provision T.sub.1 lines because its long reach requires regeneration-signal boosting only every 12,000 feet, compared with every 4,000 feet for other T1 provisioning techniques. In fact, HDSL's ability to simplify and cheapen T1 deployment has made HDSL by far the most established of the DSL technology family.
As an inexpensive and flexible replacement for leased T.sub.1 lines, the HDSL2 standards are eagerly awaited by the DSL industry. HDSL2 replaces the aging HDSL standard that required two copper pairs. HDSL2 uses only one copper pair and is potentially rate adjustable. HDSL2, which is being developed within the framework of the American National Standards Institute (ANSI, New York), promises to make HDSL more compelling in two ways. While HDSL was a proprietary technique, i.e., modems at the central office (CO) and the customer premises had to come from the same vendor, HDSL2 will be an interoperable standard in which modems can be mixed. Perhaps the biggest selling point of HDSL2, however, is that it can use one pair of copper wires instead of HDSL's two. Network service providers thus have a choice. HDSL and one-pair HDSL2 have about the same reach, while two-pair HDSL2 adds as much as another 4,000 feet of reach, depending on the gauge of copper and other conditions. Hoping to propel the new DSL technology into the business arena, eight chip makers and OEMs have formed a consortium for the HDSL2 standard.
A typical HDSL2 transceiver transmit path includes a framer, a data pump with an analog interface for coupling to the twisted-pair line. In the transmit function, the framer accepts a digital signal and outputs to the data pump a serial digital signal that includes the data payload plus an HDSL2 overhead. In the receive function, the framer receives HDSL frames from the data pump.
The data pump includes a transceiver and an analog front end for receiving the HDSL frames serially from the framer. The transceiver converts the HDSL frames into a transmit signal by first converting the HDSL frames into symbols. Typically, a modulator, such as a trellis code modulator (TCM) encodes the symbols into a pulse amplitude modulation (PAM) signal. The signal is further processed to condition and filter the PAM signal. The analog front end provides pulse shaping to analog signals. This process is reversed in the receive channel with echo cancellation provided to cancel most of the echoed transmit signal.
As mentioned, the analog front end includes a transmit and a receive channel. In the transmit channel, the analog front end receives a pulse width modulated signal stream from the transceiver. A switched capacitor circuit filter shapes the transmitted signal to meet specific spectral templates. The receive channel consists of an automatic gain control (AGC) stage and an analog-to-digital (A/D) converter. The AGC stage sets the amplitude to the optimum level to prevent saturation of the A/D converter.
The electronic equipment and communications systems taking advantage of HDSL2 technology may also benefit from the use of effective modulation techniques. Delta-sigma (.DELTA.-.SIGMA.) modulation uses the concept of oversampling and digital signal processing to achieve a high degree of accuracy in analog-to-digital (A/D) and digital-to-analog (D/A) conversion. One advantage that .DELTA.-.SIGMA. data converters offer is that they allow implementation of very high precision A/D and D/A converters without requiring stringent matching of fabricated components. This is accomplished by trading resolution for data processing speed--by sampling the data at much higher frequency than the frequency of the signal (referred to as oversampling ratio). This technique pushes the effects of low resolution components into high frequency noise band.
As the bandwidth of the signal increases, it becomes difficult to maintain a high oversampling ratio because digital and analog signal processing must be performed at very high clock frequencies. Higher clock frequency operation is sometimes technology-limited, and may lead to higher circuit power consumption.
One way to achieve a comparable level of performance from .DELTA.-.SIGMA. modulators at lower oversampling ratios is to reduce the amount of quantization error in the feedback. This may be performed using a multi-bit feedback D/A converter (DAC) in the modulator loop. The reduction in the signal-to-noise (S/N) ratio due to reducing the oversampling ratio can be compensated for by contemporaneously reducing the quantization error in the feedback loop. This technique becomes very attractive for wide band communication signal.
However, a large part of the appeal of .DELTA.-.SIGMA. modulators is the use of single bit quantizer, which is linear by design. When a multi-bit quantizer is introduced, it must be linear at least to the level of linearity required in the overall system. In an A/D converter (ADC) implementation, the quantizer is implemented in the analog feedback loop of the modulator. In a DAC implementation, the actual modulator feedback quantizer is digital, and following the modulator a quantizer is implemented as an analog DAC followed by analog noise filters.
In high-linearity DAC applications where a multi-bit .DELTA.-.SIGMA. modulator will be used, the multi-bit analog DAC will have limited linearity based on analog component matching. These analog mismatches can be corrected by traditional randomization techniques on the analog component to average out and minimize mismatch effects, or by the trimming of analog components, which is expensive and undesirable.
If these analog non-linearities can be replicated in the digital quantizer of the .DELTA.-.SIGMA. modulator loop, the effect of these nonlinearities can be eliminated and the overall system will behave as if it has ideal digital and analog DACs. It would be desirable to have a digital calibration technique for the digital feedback quantizer to allow precise replication of the analog nonlinearities in the digital quantizer.
It can be seen then that there is a need for a method and apparatus for calibrating a .DELTA.-.SIGMA. modulator quantizer to substantially replicate relative mismatches in the multi-bit digital-to-analog converter in the transmit channel of the modulator. The present invention provides a solution to these and other shortcomings of the prior art, and offers additional advantages over the prior art.